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Modeling effects of electron-velocity overshoot in a MOSFET

J.B. Roldan, F. Gamiz, J.A. Lopez-Villanueva, J.E. Carceller

1997
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IEEE Transactions on Electron Devices
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A simple analytical expression to account for electron-velocity overshoot effects on the performance of very short-channel MOSFET's has been obtained. This new model can be easily included in circuit simulators of systems with a huge number of components. The influence of temperature and low-field mobility on the increase of MOSFET transconductance produced by electron-velocity overshoot as channel lengths are reduced can be easily taken into account in our model. The accuracy of this model has
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... been verified by reproducing experimental and simulated data reported by other authors. I. INTRODUCTION N ONLOCAL effects are becoming more and more prominent as MOSFET dimensions shrink to the deepsubmicrometer regime. Velocity overshoot is one of the most important effects from the practical point of view as it is directly related with the increase of current drive and transconductance experimentally observed in short-channel MOSFET's [1]-[4]. Some authors [1]-[4] have shown that experimental measurements of submicron MOSFET transconductances are higher than the theoretical maximum transconductance that can be reached in the case where electrons drift in equilibrium with the lattice, with their electron velocity being limited by the saturation velocity [1], [5] . This result has been shown for channel lengths under 0.15 m, which means that the electron velocity along the channel is higher than the saturation one; therefore, if velocity overshoot can be controlled, the performance of very short-channel MOSFET's can be improved with respect to the performance of long-channel transistors. This effect is also reasonably well understood from the theoretical point of view [6], [7] . It has been shown that an electric field step causes the electron velocity to overshoot the value that corresponds to the higher field for a period shorter than the energy relaxation time [6], [7] (the time needed by the electron to once again reach equilibrium with the lattice [8]). Therefore, as the longitudinal electric field increases, the electron gas starts to be in disequilibrium with the lattice [9] . There is an insufficient number of phonon-scattering events experienced by the electron during its flight, with the result that electrons can be accelerated to velocities higher than the saturation velocity, thus approaching ballistic transport conditions. This effect is due to the nonequivalence of momentum and energy-relaxation times and can be observed for a period shorter than the energy relaxation time. Hence, overshoot is Manuscript

doi:10.1109/16.568047
fatcat:2kraek7x3zhy5nrpohavo55ixi